An Interactive Study: Laws of Conservation of Mass and Definite Proportions

Summary

Dalton's Playhouse is an interactive concept simulation meant to convey fundamental concepts in chemistry, including the law of definite proportions, the law of multiple proportions, and the law of conservation of mass.

This activity seeks to have students model the process of science by recording quantitative and qualitative attributes of reactants and products in three separate experiments with the purpose of examining the relationship between the original reactant(s) to the final product(s). Students record the mass and volume of reactants and products and independently calculate mathematical relationships between the reactant(s) and product(s). They also record their observations of any physical changes that occur.

Learning Goals

After calculating relationships, as well as making observations from the simulation, students are more familiar with the application of law of definite proportions, the law of multiple proportions, and the law of conservation of mass. The key goal of this activity is that students have all of the information given to them through the simulation but they need to make sense of it.

By using this simulation interactively, students see first hand how measurement and observation are used as evidence to build laws in science. This simulation shows ways in which early experimentation by one scientist may lead to new discoveries made by later scientists. Joseph Priestley's experiments led Lavoisier's to think about matter and elements in a new way. These experiments helped to lay the foundation for Dalton's theories on atomic structure.

Context for Use

This activity may be carried out individually or in small groups during lecture class. It takes about a half hour. This method of instruction is designed for a class lecture with one computer and overhead used by the instructor.

Students can also use the laboratory simulation in a lab or at home where they can personalize the simulation by adding their own name, entering notes and respond to the Concepts section.

Description and Teaching Materials

Background:
Dalton's Playhouse is an interactive concept simulation meant to convey fundamental concepts in chemistry, including the law of definite proportions, the law of multiple proportions, and the law of conservation of mass. The Playhouse has 4 main parts:

The first sequence, Priestley, simulates an 18th century laboratory in which Joseph Priestley may have carried out his groundbreaking experiments leading to the discovery of oxygen. The lab set provides a measured allotment of calx (Mercuric Oxide) in a sealed system. As the HgO is heated, the solid, crystalline calx liberates liquid mercury and O2 gas, which collects in an adjacent water-filled container in which students can see the production of the gas as displacement of the liquid. The simulation provides tools to measure volume and mass and allows experimentation with three different amounts of calx.

The second sequence, Lavoisier, simulates a 19th century lab in which Antoine Lavoisier may have carried out his experiments combining Oxygen with the gas phlogiston (later renamed Hydrogen). The set-up contains two gas-filled columns with floating septa. As the reactants are consumed in the central flame, volumes within these containers decrease. The product then collects in a third septa-sealed column. Students can measure mass and volume of all reactants and products to see how these measurements change upon reaction.

The third sequence, Diamond, simulates a 19th century lab in Stanislaw Maillard may have carried out his classic experiment in which he burned diamonds. Students can burn equal weights of diamond and graphite and measure the mass and volume of reactants produced upon combustion.

The fourth sequence, Concepts, provide concept-driving questions to motivate students to explore all aspects of the simulation.

As a precursor to this activity, the lecture should include the historical perspective of philosophers' and scientists' ideas of the composition of matter. The Visionlearning web page on Matter takes this perspective.

Index cards or notebook paper

How To:Dalton's Playhouse
Introduce Dalton's Playhouse by explaining that scientists use mathematics and observation to understand relationships between what they begin with and what they end up with. Explain that the simulation examines the way in which experimentation and theory building are an ongoing process within the scientific community. Explain that the scale and volume tools will help students to make measurements but that they also need to make observations while conducting the simulated experiments. Students are not told how to find the relationship between reactant(s) and product(s) but only to record what they measure and observe.

The key aspect of this activity is that students have all of the information given to them through the simulation but they need to make sense of it.
Begin simulation. Read the introduction to the whole class. Explain the scale and volume tool. Note that volume tools will only display measurements of gases and the scale tool works for all material.

Priestley's Laboratory
Read introduction to class and then proceed to the laboratory. Explain the laboratory environment showing the instruments and highlighting the nature of the calx (it's a solid). Ask students to create some kind of table for their measurements that includes a column for reactant and product(s). Show the original measurements of the calx by choosing the scale and dragging it over the calx. Show that the other instruments read "0." Burn 100 grams of calx and then drag the scale over the new reactant and new products. I find it helpful to read the new measurements out loud and to repeat the burning of calx for each amount (100g, 200 g, and 216.59g).

After students have created a table with recorded amounts, ask them to calculate the relationship between the values (in any way they want) to see if there are any consistent patterns derived from the amounts.

Students usually see that the two products add up to the original amount of the reactant. Some students also calculate the ratio of the products to the reactant and find that the liquid remaining (mercury) is always 93% (0.926) of the original amount and that the gas is 7% (0.074) of the original amount.

Be sure to have students also describe what they see happening during the burning of calx and the nature of the resulting products.
This discovery should lead to a discussion about the fact that some substances are composed of discrete amounts of two or more other substances and the law of conservation of mass.

Instructors may also want to build off of what is explained in the Visionlearning chapter:
"Priestley had observed that it does not just turn into mercury; it actually breaks down into two substances when it is heated, liquid mercury and a strange gas. Priestley carefully collected this gas in glass jars and studied it. After many long days and nights in the laboratory, Priestley said of the strange gas, "what surprised me more than I can well express was that a candle burned in this air with a remarkably vigorous flame." Not only did flames burn strongly in this gas, but a mouse placed in a sealed container of this gas lived for a longer period of time than a mouse placed in a sealed container of ordinary air. Priestley's discovery revealed that substances could combine together or break apart to form new substances with different properties. For example, a colorless, odorless gas could combine with mercury, a silver metal, to form mercury calx, a red mineral."
Lavoisier's Laboratory
Follow the same procedure as above except use volume tool to measure the amount of gas in liters. Be sure to advise students to watch the gas-filled columns with floating septa.

This experimental simulation allows for students to determine the law of definite proportions as the phlogiston (hydrogen) always combines with the oxygen in a 2:1 ratio. Advise students to think about the proportion of remaining amounts of phlogiston and oxygen in relation to the original amounts and in relation of each other. For example, ask them:
- How much of the Phlogiston and Oxygen were used in the reaction?
- What is the relationship of the amounts to each other?

Burning Diamonds (Who's crazy enough to do that?)
Students can burn equal weights of diamond and graphite and measure the mass and volume of reactant produced upon combustion. Discuss the element carbon and the gas present - oxygen. Follow the same procedure as above using the scale and/or volume instruments. Students will see that elements combine in specific defined ratios in chemical reactions.

Teaching Notes and Tips

Some students are comfortable with ratios and will understand this activity right away. Class discussion follows each laboratory simulation (see assessment) and this allows students to volunteer their findings, but the instructor needs to recap the findings and the implication of the findings a few times in different ways. There is no need to introduce the "laws" right away - just the concepts.

Assessment

Class discussion follows each laboratory simulation and appropriate concepts, as described in the laws, are discussed. Index cards or notebook paper are collected after the activity is completed. Students are given points for working through the mathematics and writing down observations.